Claude Tools and Function Calling: Developer Guide

Claude's tool use feature allows you to give the AI access to external functions, dramatically expanding what it can accomplish. This guide covers implementation and best practices.

What is Tool Use?

Tool use allows Claude to:

• Call functions you define
• Access external data and services
• Perform actions in the real world
• Work with structured data

How It Works

1. Define Tools

Describe available functions with:

• Name and description
• Input parameters with types
• Expected output format

2. Claude Decides

When processing a request, Claude:

• Analyzes what's needed
• Selects appropriate tool(s)
• Formats the tool call

3. You Execute

Your application:

• Receives the tool call
• Executes the function
• Returns results to Claude

4. Claude Responds

Claude incorporates tool results into its response.

Tool Definition Format

Tools are defined as JSON schemas:

{ "name": "get_weather", "description": "Get current weather for a location", "input_schema": { "type": "object", "properties": { "location": { "type": "string", "description": "City and country" }, "units": { "type": "string", "enum": ["celsius", "fahrenheit"] } }, "required": ["location"] } }

Best Practices

Clear Descriptions

Write descriptions that help Claude understand when to use each tool:

• What the tool does
• When it's appropriate
• What inputs it needs
• What output to expect

Error Handling

Plan for tool failures:

• Return clear error messages
• Allow Claude to retry or adapt
• Provide fallback behaviors

Security

Implement safeguards:

• Validate all inputs
• Limit tool capabilities appropriately
• Log tool usage
• Implement rate limits

Common Patterns

Data Retrieval

Tools that fetch information:

• Database queries
• API calls
• File reads
• Web searches

Actions

Tools that do things:

• Send messages
• Update records
• Create files
• Trigger workflows

Calculations

Tools for precise computation:

• Mathematical operations
• Data processing
• Format conversions

Example Implementation

Weather Assistant

1. Define get_weather tool
2. User asks "What's the weather in Paris?"
3. Claude calls get_weather("Paris, France")
4. Your code fetches weather data
5. Return: {"temp": 18, "conditions": "partly cloudy"}
6. Claude: "It's currently 18°C and partly cloudy in Paris."

Debugging Tips

• Log all tool calls and responses
• Test tools independently first
• Use verbose mode during development
• Monitor for unexpected tool selections

Tool use transforms Claude from a text generator into an action-taking assistant.